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>Chapter 47. Logical Decoding</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN"
>47.6. Logical Decoding Output Plugins</A
></H1
><P
> An example output plugin can be found in the
<A
HREF="test-decoding.html"
> <TT
CLASS="FILENAME"
>contrib/test_decoding</TT
>
</A
>
subdirectory of the PostgreSQL source tree.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOGICALDECODING-OUTPUT-INIT"
>47.6.1. Initialization Function</A
></H2
><P
> An output plugin is loaded by dynamically loading a shared library with
the output plugin's name as the library base name. The normal library
search path is used to locate the library. To provide the required output
plugin callbacks and to indicate that the library is actually an output
plugin it needs to provide a function named
<CODE
CLASS="FUNCTION"
>_PG_output_plugin_init</CODE
>. This function is passed a
struct that needs to be filled with the callback function pointers for
individual actions.
</P><PRE
CLASS="PROGRAMLISTING"
>typedef struct OutputPluginCallbacks
{
LogicalDecodeStartupCB startup_cb;
LogicalDecodeBeginCB begin_cb;
LogicalDecodeChangeCB change_cb;
LogicalDecodeCommitCB commit_cb;
LogicalDecodeMessageCB message_cb;
LogicalDecodeFilterByOriginCB filter_by_origin_cb;
LogicalDecodeShutdownCB shutdown_cb;
} OutputPluginCallbacks;
typedef void (*LogicalOutputPluginInit) (struct OutputPluginCallbacks *cb);</PRE
><P>
The <CODE
CLASS="FUNCTION"
>begin_cb</CODE
>, <CODE
CLASS="FUNCTION"
>change_cb</CODE
>
and <CODE
CLASS="FUNCTION"
>commit_cb</CODE
> callbacks are required,
while <CODE
CLASS="FUNCTION"
>startup_cb</CODE
>,
<CODE
CLASS="FUNCTION"
>filter_by_origin_cb</CODE
>
and <CODE
CLASS="FUNCTION"
>shutdown_cb</CODE
> are optional.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOGICALDECODING-CAPABILITIES"
>47.6.2. Capabilities</A
></H2
><P
> To decode, format and output changes, output plugins can use most of the
backend's normal infrastructure, including calling output functions. Read
only access to relations is permitted as long as only relations are
accessed that either have been created by <TT
CLASS="COMMAND"
>initdb</TT
> in
the <TT
CLASS="LITERAL"
>pg_catalog</TT
> schema, or have been marked as user
provided catalog tables using
</P><PRE
CLASS="PROGRAMLISTING"
>ALTER TABLE user_catalog_table SET (user_catalog_table = true);
CREATE TABLE another_catalog_table(data text) WITH (user_catalog_table = true);</PRE
><P>
Any actions leading to transaction ID assignment are prohibited. That, among others,
includes writing to tables, performing DDL changes, and
calling <TT
CLASS="LITERAL"
>txid_current()</TT
>.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOGICALDECODING-OUTPUT-MODE"
>47.6.3. Output Modes</A
></H2
><P
> Output plugin callbacks can pass data to the consumer in nearly arbitrary
formats. For some use cases, like viewing the changes via SQL, returning
data in a data type that can contain arbitrary data (e.g., <TT
CLASS="TYPE"
>bytea</TT
>) is
cumbersome. If the output plugin only outputs textual data in the
server's encoding, it can declare that by
setting <TT
CLASS="LITERAL"
>OutputPluginOptions.output_type</TT
>
to <TT
CLASS="LITERAL"
>OUTPUT_PLUGIN_TEXTUAL_OUTPUT</TT
> instead
of <TT
CLASS="LITERAL"
>OUTPUT_PLUGIN_BINARY_OUTPUT</TT
> in
the <A
HREF="logicaldecoding-output-plugin.html#LOGICALDECODING-OUTPUT-PLUGIN-STARTUP"
>startup
callback</A
>. In that case, all the data has to be in the server's encoding
so that a <TT
CLASS="TYPE"
>text</TT
> datum can contain it. This is checked in assertion-enabled
builds.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-CALLBACKS"
>47.6.4. Output Plugin Callbacks</A
></H2
><P
> An output plugin gets notified about changes that are happening via
various callbacks it needs to provide.
</P
><P
> Concurrent transactions are decoded in commit order, and only changes
belonging to a specific transaction are decoded between
the <TT
CLASS="LITERAL"
>begin</TT
> and <TT
CLASS="LITERAL"
>commit</TT
>
callbacks. Transactions that were rolled back explicitly or implicitly
never get
decoded. Successful savepoints are
folded into the transaction containing them in the order they were
executed within that transaction.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> Only transactions that have already safely been flushed to disk will be
decoded. That can lead to a <TT
CLASS="COMMAND"
>COMMIT</TT
> not immediately being decoded in a
directly following <TT
CLASS="LITERAL"
>pg_logical_slot_get_changes()</TT
>
when <TT
CLASS="VARNAME"
>synchronous_commit</TT
> is set
to <TT
CLASS="LITERAL"
>off</TT
>.
</P
></BLOCKQUOTE
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-STARTUP"
>47.6.4.1. Startup Callback</A
></H3
><P
> The optional <CODE
CLASS="FUNCTION"
>startup_cb</CODE
> callback is called whenever
a replication slot is created or asked to stream changes, independent
of the number of changes that are ready to be put out.
</P><PRE
CLASS="PROGRAMLISTING"
>typedef void (*LogicalDecodeStartupCB) (struct LogicalDecodingContext *ctx,
OutputPluginOptions *options,
bool is_init);</PRE
><P>
The <TT
CLASS="LITERAL"
>is_init</TT
> parameter will be true when the
replication slot is being created and false
otherwise. <TT
CLASS="PARAMETER"
>options</TT
> points to a struct of options
that output plugins can set:
</P><PRE
CLASS="PROGRAMLISTING"
>typedef struct OutputPluginOptions
{
OutputPluginOutputType output_type;
} OutputPluginOptions;</PRE
><P>
<TT
CLASS="LITERAL"
>output_type</TT
> has to either be set to
<TT
CLASS="LITERAL"
>OUTPUT_PLUGIN_TEXTUAL_OUTPUT</TT
>
or <TT
CLASS="LITERAL"
>OUTPUT_PLUGIN_BINARY_OUTPUT</TT
>. See also
<A
HREF="logicaldecoding-output-plugin.html#LOGICALDECODING-OUTPUT-MODE"
>Section 47.6.3</A
>.
</P
><P
> The startup callback should validate the options present in
<TT
CLASS="LITERAL"
>ctx->output_plugin_options</TT
>. If the output plugin
needs to have a state, it can
use <TT
CLASS="LITERAL"
>ctx->output_plugin_private</TT
> to store it.
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-SHUTDOWN"
>47.6.4.2. Shutdown Callback</A
></H3
><P
> The optional <CODE
CLASS="FUNCTION"
>shutdown_cb</CODE
> callback is called
whenever a formerly active replication slot is not used anymore and can
be used to deallocate resources private to the output plugin. The slot
isn't necessarily being dropped, streaming is just being stopped.
</P><PRE
CLASS="PROGRAMLISTING"
>typedef void (*LogicalDecodeShutdownCB) (struct LogicalDecodingContext *ctx);</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-BEGIN"
>47.6.4.3. Transaction Begin Callback</A
></H3
><P
> The required <CODE
CLASS="FUNCTION"
>begin_cb</CODE
> callback is called whenever a
start of a committed transaction has been decoded. Aborted transactions
and their contents never get decoded.
</P><PRE
CLASS="PROGRAMLISTING"
>typedef void (*LogicalDecodeBeginCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn);</PRE
><P>
The <TT
CLASS="PARAMETER"
>txn</TT
> parameter contains meta information about
the transaction, like the time stamp at which it has been committed and
its XID.
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-COMMIT"
>47.6.4.4. Transaction End Callback</A
></H3
><P
> The required <CODE
CLASS="FUNCTION"
>commit_cb</CODE
> callback is called whenever
a transaction commit has been
decoded. The <CODE
CLASS="FUNCTION"
>change_cb</CODE
> callbacks for all modified
rows will have been called before this, if there have been any modified
rows.
</P><PRE
CLASS="PROGRAMLISTING"
>typedef void (*LogicalDecodeCommitCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn,
XLogRecPtr commit_lsn);</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-CHANGE"
>47.6.4.5. Change Callback</A
></H3
><P
> The required <CODE
CLASS="FUNCTION"
>change_cb</CODE
> callback is called for every
individual row modification inside a transaction, may it be
an <TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>,
or <TT
CLASS="COMMAND"
>DELETE</TT
>. Even if the original command modified
several rows at once the callback will be called individually for each
row.
</P><PRE
CLASS="PROGRAMLISTING"
>typedef void (*LogicalDecodeChangeCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn,
Relation relation,
ReorderBufferChange *change);</PRE
><P>
The <TT
CLASS="PARAMETER"
>ctx</TT
> and <TT
CLASS="PARAMETER"
>txn</TT
> parameters
have the same contents as for the <CODE
CLASS="FUNCTION"
>begin_cb</CODE
>
and <CODE
CLASS="FUNCTION"
>commit_cb</CODE
> callbacks, but additionally the
relation descriptor <TT
CLASS="PARAMETER"
>relation</TT
> points to the
relation the row belongs to and a struct
<TT
CLASS="PARAMETER"
>change</TT
> describing the row modification are passed
in.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> Only changes in user defined tables that are not unlogged
(see <A
HREF="sql-createtable.html#SQL-CREATETABLE-UNLOGGED"
><I
CLASS="TERM"
><TT
CLASS="LITERAL"
>UNLOGGED</TT
></I
></A
>) and not temporary
(see <A
HREF="sql-createtable.html#SQL-CREATETABLE-TEMPORARY"
><I
CLASS="TERM"
><TT
CLASS="LITERAL"
>TEMPORARY</TT
> or <TT
CLASS="LITERAL"
>TEMP</TT
></I
></A
>) can be extracted using
logical decoding.
</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-FILTER-ORIGIN"
>47.6.4.6. Origin Filter Callback</A
></H3
><P
> The optional <CODE
CLASS="FUNCTION"
>filter_by_origin_cb</CODE
> callback
is called to determine whether data that has been replayed
from <TT
CLASS="PARAMETER"
>origin_id</TT
> is of interest to the
output plugin.
</P><PRE
CLASS="PROGRAMLISTING"
>typedef bool (*LogicalDecodeFilterByOriginCB) (struct LogicalDecodingContext *ctx,
RepOriginId origin_id);</PRE
><P>
The <TT
CLASS="PARAMETER"
>ctx</TT
> parameter has the same contents
as for the other callbacks. No information but the origin is
available. To signal that changes originating on the passed in
node are irrelevant, return true, causing them to be filtered
away; false otherwise. The other callbacks will not be called
for transactions and changes that have been filtered away.
</P
><P
> This is useful when implementing cascading or multidirectional
replication solutions. Filtering by the origin allows to
prevent replicating the same changes back and forth in such
setups. While transactions and changes also carry information
about the origin, filtering via this callback is noticeably
more efficient.
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-MESSAGE"
>47.6.4.7. Generic Message Callback</A
></H3
><P
> The optional <CODE
CLASS="FUNCTION"
>message_cb</CODE
> callback is called whenever
a logical decoding message has been decoded.
</P><PRE
CLASS="PROGRAMLISTING"
>typedef void (*LogicalDecodeMessageCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn,
XLogRecPtr message_lsn,
bool transactional,
const char *prefix,
Size message_size,
const char *message);</PRE
><P>
The <TT
CLASS="PARAMETER"
>txn</TT
> parameter contains meta information about
the transaction, like the time stamp at which it has been committed and
its XID. Note however that it can be NULL when the message is
non-transactional and the XID was not assigned yet in the transaction
which logged the message. The <TT
CLASS="PARAMETER"
>lsn</TT
> has WAL
position of the message. The <TT
CLASS="PARAMETER"
>transactional</TT
> says
if the message was sent as transactional or not.
The <TT
CLASS="PARAMETER"
>prefix</TT
> is arbitrary null-terminated prefix
which can be used for identifying interesting messages for the current
plugin. And finally the <TT
CLASS="PARAMETER"
>message</TT
> parameter holds
the actual message of <TT
CLASS="PARAMETER"
>message_size</TT
> size.
</P
><P
> Extra care should be taken to ensure that the prefix the output plugin
considers interesting is unique. Using name of the extension or the
output plugin itself is often a good choice.
</P
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-OUTPUT"
>47.6.5. Functions for Producing Output</A
></H2
><P
> To actually produce output, output plugins can write data to
the <TT
CLASS="LITERAL"
>StringInfo</TT
> output buffer
in <TT
CLASS="LITERAL"
>ctx->out</TT
> when inside
the <CODE
CLASS="FUNCTION"
>begin_cb</CODE
>, <CODE
CLASS="FUNCTION"
>commit_cb</CODE
>,
or <CODE
CLASS="FUNCTION"
>change_cb</CODE
> callbacks. Before writing to the output
buffer, <CODE
CLASS="FUNCTION"
>OutputPluginPrepareWrite(ctx, last_write)</CODE
> has
to be called, and after finishing writing to the
buffer, <CODE
CLASS="FUNCTION"
>OutputPluginWrite(ctx, last_write)</CODE
> has to be
called to perform the write. The <TT
CLASS="PARAMETER"
>last_write</TT
>
indicates whether a particular write was the callback's last write.
</P
><P
> The following example shows how to output data to the consumer of an
output plugin:
</P><PRE
CLASS="PROGRAMLISTING"
>OutputPluginPrepareWrite(ctx, true);
appendStringInfo(ctx->out, "BEGIN %u", txn->xid);
OutputPluginWrite(ctx, true);</PRE
><P>
</P
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